Electrolytic treatment of metal



ELECTROLYTIC TREATMENT OF METAL SURFACES Richard M. Wick, Allentown, Pa., assignor to Bethlehem Steel Company, a corporation of Pennsylvania No Drawing. Application July 20, 1951,

Serial No. 237,849

7 Claims. (Cl. 204-56) This invention relates to the protective treatment of metal surfaces, and more particularly to the cathodic treatment of metal surfaces, such as those of iron, steel, zinc, or other base metals, in a solution containing trivalent and hexavalent chromium alongwith boric acid or salts of boric acid.

The principal object of this invention is to develop an improved paint adhering film on the surface of base metals such as iron, steel and zinc.

Another object is to inhibit underfilm corrosion in painted metal articles.

A further object is to form a protective film on articles having base metal surfaces.

Another object is to increase the efficiency of formation of chromium-containing colloidal films on metal.

It is often desirable, before painting metal surfaces, to apply a thin intermediate film to a metal surface, such that when paint is later applied there will be an adherent bond between the paint and the metal. Frequently, when paint is applied directly to a metal surface, the paint will strip, peel or chip after short periods of wear or exposure. To enhance the adherence of paint to metal, numerous metal surface films have been developed, notably films containing phosphates, which retain the paint uniformly throughout the surface of the metal. These intermediate, paint-bonding films, are generally applied to the metal by immersion or electrolysis in aqueous solutions.

Some paint-bonding films have been found to have sufficient anti-corrosive properties in mildly corrosive atmospheres, so that the film itself may assume the nature of a protective coating on metal. Even when steel or other basis stock is coated with one of the so-called protective coating metals, as tin or zinc, it is oftentimes desirable to further coat the surface with a non-metallic film, other than paint, to prevent incipient corrosion.

In the production of films on metals by means of cathodic electrolysis from solutions containing chromium, attempts have been made at film formation from solutions containing chromic acid (C103) alone. The films obtained are thin, non-uniform and inadequate from the standpoint of corrosion resistance. Very good films have been produced by cathodic electrolysis from solutions containing both chromic acid and boric acid.

I have found that when using a chromic acid-boric acid electrolyte, said electrolyte being substantially free of chromium sulfate if part of the hexavalent chromium, present as chromic acid, is reduced to trivalent chromium, prior to electrolysis, rate of film formation is increased by a considerable amount,- and the resultant film has improved corrosion resistance over films formed from electrolyte solutions of chromic and boric acids alone. Reduction of the hexavalent chromium may be obtained by adding to the electrolyte, prior to electrolysis, an organic material capable of being oxidized by the hexavalent chromium, for example, glycerine. The film formed by my invention is colloidal in nature and is believed to consist of a hydrated oxide of chromium. This hydrated oxide contains both hexavalent and trivalent nited States Patent ICC chromium, some of the Cr having been reduced in the bath to Cr The amount of Cr present in the bath is controlled by the amount of reducing material added. By this easy and practical method of control, good results are obtained with formation in the bath of trivalent chromium in amounts up to approximately 18% based on total chromium in solution. When, in the presence of chromic acid, trivalent chromium is formed in solution, it associates with a part of the remaining chromic acid to form a chromium chromate. The composition of chromium chromate is variable, but for practical purposes may be taken to be Cr(HCrO4)3. The chromic acid not thus combined as chromium chromate is present as free chromic acid. It is preferable that not more than of the original chromic acid be acted upon by the organic reducing material, thus at least 25% of the original chromic acid remains in the bath as free chromic acid. As the amount of trivalent chromium in the bath is increased through reduction of the hexavalent chromium, the amount of boric acid present may be correspondingly reduced. Boric acid, which has a catalytic action in promoting the desired film formation, may be replaced in the bath by salts of boric acid such as sodium tetraborate, calcium metaborate and potassium tetraborate, the salts of the acid having an equivalence of function to that of boric acid. In place of glycerine, any other suitable organic agent may be used in developing formation of trivalent chromium which does not tend to form undesirable by-products in solution. Examples of organic agents capable of efficiently reducing chromium from the hexavalent to the trivalent state are sucrose, ethyl alcohol and acetone. The presence of trivalent chromium in the electrolytic bath apparently has an accelerating action on the rate of film formation, and the ratio of Cr" to Cr in the film is such that superior corrosion properties are developed when the coating is used as an under-paint film, or when used alone in mildly corrosive atmospheres.

As one example of the manner in which a reduced chromium bath may be prepared, I formed an aqueous solution of the following ingredients:

G./l. Chromic acid (CrOs) 295 Boric acid (HsBOs) 20 Glycerine 7.5

As there is some heat evolved during the reaction, the materials were added slowly to prevent loss, and upon cooling to room temperature the solution was ready for use as an electrolytic coating bath. At this point some of the hexavalent chromium had been converted to trivalent chromium by the reducing action of the glycerine. A test of the solution indicated that 18.5 grams per liter of Cr were now present in the solution. It should be noted that the Cr produced in this example is well within the recommended limit of 18% of the total chromium in solution.

The method of using an organic material ito reduce hexavalent chromium is the preferred one in securing the proper amount of trivalent chromium in solution, prior to electrolyzing; however, my invention is not restricted to this particular aspect, for trivalent chromium i metals by my method is obtained under a wide range of operating conditions. For example, when corrodible ferrous metals, or metals having a surface coating of a corrodible base metal such as zinc, are treated cathodically With direct current in a bath of chromic acid, boric acid and some trivalent chromium (formed by partial reduction of chromic acid), the original addition of chromic acid, calculated as CrOs, may range from 100 to 400 grams per liter, and the boric acid, or salts of boric acid, may be present in quantities of from 8 grams per liter to saturation. The amount of glycerine, or other organic reducing agent added is determined empirically and is governed by the amount of trivalent chromium one desires to form in the bath, as well as by the nature of the reducting agent itself. Within the range given above for CrOs, i. e., 100 to 400 grams per liter, it is permissible to form trivalent chromium in the bath in quantities ranging from 5 to approximately 40 grams per liter. Satisfactory temperatures were found to be included in the range from 20 C. to 45 C., while the current density may be applied within the limits of 50 to 300 amperes per square foot. The time of treatment is controlled by the type of film desired; i. e., whether the film is to be used for improvement in paint adhesion and inhibition of under film (paint film) corrosion, or as a protective film per se. When a protective film is desired, a greater film thickness is required, hence a longer electrolyizing period is necessary, as may be determined by test.

An example of a satisfactory aqueous electrolyte with normal operating conditions is given below.

Chromic acid (CrOs) g./l 275 Boric acid (I-IzBOa) g./l 20 Cr produced by addition of glycerine g./l 18 Current density amp./sq.ft. 200 Temperature /C 24 Time seconds 13 The above bath represents a cooled electrolyte ready for use, prepared, as previously described, by the slow addition of glycerine to a solution of chromic acid and boric acid. The glycerine converts part of the Cr" to nr As there will be some depletion of the electrolyte during use, the electrolyte should be tested from time to time so that additions can be made and a properly balanced bath maintained.

The above-mentioned bath, under the operating conditions given therewith, was used in a series of actual tests, and it produced an excellent chromium chromate film on electrolytic zinc-plated steel sheet and an blackplate.

My method of producing a hydrated oxide of chromium film on metal surfaces, such as steel (blackplate), zinccoated sheets, or other base metal surfaces, is particularly advantageous where numerous large sheets must be processed, for the formation of trivalent chromium in the bath prior to electrolyzing increases the efiiciency of the treatment by as much as 100% over any previously known method for the production of a similar type of chromium bearing colloidal film. Besides, as has been mentioned, increased corrosion resistance characteristics are obtained.

My method of treatment can be successfully applied to any base metal, and it has particular utility in respect to zinc, tin, magnesium, cadmium and the corrodible ferrous metals. By base metals I mean to include all metals which corrode readily, as distinguished from the noble metals and the non-corrodible alloys such as stainless steel.

I claim:

1. The process of forming a non-metallic coating on the surface of a metal article which comprises electrolyzing a metal article having a surface material of corrodible base metal in an aqueous solution consisting essentially of chromic acid (100 to 400 grams per liter), a compound of the class consisting of boric acid and salts of boric acid, said compound being present in an amount of from 8 grams per liter to saturation, and chromium chromate in which the trivalent chromium is present in an amount between 2 per cent and 18 per cent of the total chromium in solution, said metal article acting as the cathode.

2. The process of forming a non-metallic coating on the surface of a metal article which comprises electrolyzing a zinc-coated article in an aqueous solution consisting essentially of chromic acid (100 to 400 grams per liter), a compound of the class consisting of boric acid and salts of boric acid, said compound being present in an amount of from 8 grams per liter to saturation, and chromium chromate in which the trivalent chromium is present in an amount between 2 per cent and 18 per cent of the total chromium in solution, said metal article acting as the cathode.

3. The process of forming a non-metallic coating on the surface of a metal article which comprises electrolyzing a corrodible ferrous article in an aqueous solution consisting essentially of chromic acid (100 to 400 grams per liter), a compound of the class consisting of boric acid and salts of boric acid, said compound being present in an amount of from 8 grams per liter to saturation, and chromium chromate in which the trivalent chrominm is present in an amount between 2 per cent and 18 per cent of the total chromium in solution, said metal xt'icle acting as the cathode.

4. The process of forming a non-metallic coating on the surface of a metal article which comprises electrolyzing a tin-coated article in an aqueous solution consisting essentially of chromic acid (100 to 400 grams per liter), a compound of the class consisting of boric acid and salts of boric acid, said compound being present in an amount of from 8 grams per liter to saturation, and chromium chromate in which the trivalent chromium is present in an amount between 2 per cent and 18 per cent of the total chromium in solution, said metal article acting as the cathode.

5. The process of forming a non-metallic coating on the surface of a metal article which comprises electrolyzing a metal article having a surface material of corrodible base metal in an aqueous solution consisting essentially of chromic acid (100 to 400 grams per liter), boric acid (8 grams per liter to saturation) and chromium chromate in which the trivalent chromium is present in an amount between 2 per cent and 18 per cent of the total chromium in solution, said metal article acting as the cathode.

6. The process of forming a non-metallic coating on the surface of a metal article which comprises electrolyzing a zinc-coated article in an aqueous solution consisting essentially of chromic acid (100 to 400 grams per liter),

' boric acid (10 grams per liter to saturation) and chromium chromate in which the trivalent chromium is pres ent in an amount between 2 per cent and 18 per cent of the total chromium in solution, said metal article acting as the cathode.

7. The process of forming a non-metallic coating on the surface of a metal article which comprises electrolyzing a metal article having a surface material of corrodible base metal in an aqueous solution consisting essentially of chromic acid (100 to 400 grams per liter), boric acid (8 grams per liter to saturation) and chromium chromate in which the trivalent chromium is present in an amount between 2 per cent and 18 per cent of the total chromium in solution, said metal article acting as the cathode, at a current density of from to 300 amperes per square foot for not less than 10 seconds.

References Cited in the file of this patent UNITED STATES PATENTS 1,827,247 Mason Oct. 13, 1931 1,838,777 McCullough ct a1 Dec. 29, 1931 1,853,323 Schulein Apr. 12, 1932 2,063,197 Sehneidewind Dec. 8, 1936 2,635,993 Snavcly Apr. 21, 1953 FOREIGN PATENTS 254,757 Great Britain Oct. 6, 1927 OTHER REFERENCES Principles of Electroplating and Electroforming, by Blum et al., second edition (1930), page 293. t 

1. THE PROCESS OF FORMING A NON-METALLIC COATING ON THE SURFACES OF A METAL ARTICLE WHICH COMPRISES ELECTROLYZING A METAL ARTICLE HAVING A SURFACE MATERIAL OF CORRODIBLE BASE METAL IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF CHROMIC ACID (100 TO 400 GRAMS PER LITER), A COMPOUND OF THE CLASS CONSISTING OF BORIC ACID AND SALTS OF BORIC ACID, SAID COMPOUND BEING PRESENT IN AN AMOUNT OF FROM 8 GRAMS PER LITER TO SATURATION, AND CHROMIUM CHROMATE IN WHICH THE TRIVALENT CHROMIUM IS PRESENT IN AN AMOUNT BETWEEN 2 PER CENT AND 18 PER CENT OF THE TOTAL CHROMIUM IN SOLUTION, SAID METAL ARTICLE ACTING AS THE CATHODE. 